On this particular Thursday, something was moving quietly through the ionosphere many miles above the surface of the planet; several somethings in fact, several dozen huge yellow chunky slablike somethings, huge as office buildings, silent as birds. They soared with ease, basking in electromagnetic rays from the star Sol, biding their time, grouping, preparing.

The planet beneath them was almost perfectly oblivious of their presence, which was just how they wanted it for the moment. The huge yellow somethings went unnoticed at Goonhilly, they passed over Cape Canaveral without a blip, Woomera and Jodrell Bank looked straight through them — which was a pity because it was exactly the sort of thing they'd been looking for all these years.

The only place they registered at all was on a small black device called a Sub-Etha Sens-O-Matic which winked away quietly to itself. It nestled in the darkness inside a leather satchel which Ford Prefect wore habitually round his neck. The contents of Ford Prefect's satchel were quite interesting in fact and would have made any Earth physicist's eyes pop out of his head, which is why he always concealed them by keeping a couple of dog-eared scripts for plays he pretended he was auditioning for stuffed in the top. Besides the Sub-Etha Sens-O-Matic and the scripts he had an Electronic Thumb — a short squat black rod, smooth and matt with a couple of flat switches and dials at one end; he also had a device which looked rather like a largish electronic calculator. This had about a hundred tiny flat press buttons and a screen about four inches square on which any one of a million "pages" could be summoned at a moment's notice. It looked insanely complicated, and this was one of the reasons why the snug plastic cover it fitted into had the words Don't Panic printed on it in large friendly letters. The other reason was that this device was in fact that most remarkable of all books ever to come out of the great publishing corporations of Ursa Minor — The Hitch Hiker's Guide to the Galaxy. The reason why it was published in the form of a micro sub meson electronic component is that if it were printed in normal book form, an interstellar hitch hiker would require several inconveniently large buildings to carry it around in.

Beneath that in Ford Prefect's satchel were a few biros, a notepad, and a largish bath towel from Marks and Spencer.

The Hitch Hiker's Guide to the Galaxy has a few things to say on the subject of towels.

A towel, it says, is about the most massively useful thing an interstellar hitch hiker can have. Partly it has great practical value — you can wrap it around you for warmth as you bound across the cold moons of Jaglan Beta; you can lie on it on the brilliant marble-sanded beaches of Santraginus V, inhaling the heady sea vapours; you can sleep under it beneath the stars which shine so redly on the desert world of Kakrafoon; use it to sail a mini raft down the slow heavy river Moth; wet it for use in hand-to-hand-combat; wrap it round your head to ward off noxious fumes or to avoid the gaze of the Ravenous Bugblatter Beast of Traal (a mindboggingly stupid animal, it assumes that if you can't see it, it can't see you — daft as a bush, but very ravenous); you can wave your towel in emergencies as a distress signal, and of course dry yourself off with it if it still seems to be clean enough.